Hydrologic cycle comprises the movement of water on Earth: It evaporates from the oceans, precipitates back to the surface, flows in the form of runoff to bodies of water, and infiltrates and is stored underground. A warming climate affects this cycle and therefore all life on the planet. Steady warming increases the atmosphere’s moisture-holding capacity, altering the hydrologic cycle and the characteristics of precipitation, most notably intensity. Changes in the global rate and distribution of precipitation may have a greater direct effect on human well-being and ecosystem dynamics than do changes in temperature. While temperature increases in a linear fashion, scientific research indicates that the hydrologic cycle may change exponentially, according to David Easterling of the U.S. National Climatic Data Center.
Atmospheric moisture increases rapidly as temperatures rise; over the United States and Europe, atmospheric moisture increased by 10 to 20 percent from 1980 to 2000. Studies of tree rings have provided a reconstruction of precipitation variability in the high mountains of northern Pakistan going back one thousand years that indicates a large-scale intensification of the hydrologic cycle coincided with the onset of industrialization and global warming. The unprecedented amplitude of this change strongly suggests a human role. This study suggests that an unprecedented intensification of the hydrologic cycle in western Central Asia occurred during the twentieth century.
In many places, the daily weather increasingly was becoming a question of drought or deluge. Paradoxically, the intensification of the hydrologic cycle due to a warming climate may provoke both drought and deluge sometimes alternately in the same location. The behavior of this cycle also figures into a lively debate regarding whether a generally warming climate may cause hurricanes to intensify, as well as the impact of climate change on the worldwide spread of deserts, which may contribute to the number of “environmental refugees,” and on disaster relief generally.